MAR-23: Implemented tests for the database role:

- Updated test playbook to set-up backup_server as separate play in order to
avoid skipping of backup_client role.
- Updated test playbook to correctly set-up a single backup client.
- Do not manage permissions on /srv/ directory in database role. This can
interfere with backup client access.
- Do not manage permissions on /srv/ directory in ldap_server role. This can
interfere with backup client access.
- Implemented common tests.
- Implemented tests related to database backup.